There were obvious advantages to such a system: no ambiguity about where authority lay, no time wasted in legislative debates, no need to cobble coalitions together to get things done. Yet Louis’s France, Europe’s most powerful state, was fought to a standstill by England and the Netherlands — a constitutional monarchy (after the Glorious Revolution of 1688) and a republic, with a combined population only a fraction as large.

And in the Anglo-French wars that followed, France generally got the worst of it, while sliding ever deeper into fiscal crisis — a crisis that eventually helped precipitate the French Revolution.

Why was an absolute monarchy weaker, in practice, than quarrelsome republics? One reason was that the very absence of limits on the ruler undermined French credibility: Whatever the king might promise, he could always change his mind. Not incidentally, France repeatedly defaulted on its debt, while post-1688 England, its king effectively constrained by Parliament, never did. As a result, England was much more successful at wartime borrowing, and paid much lower interest rates.

Which brings us, as all things do these days, to Donald Trump — a man who has evident contempt for the rule of law and who, like Louis, sees no distinction between loyalty to the nation and loyalty to himself. The main difference is that Louis seems to have at least tried to understand the issues.

On Friday night, something unprecedented happened: The U.S. government shut down temporarily even though the same party controls both Congress and the White House. Why? Because when it comes to Trump, a deal isn’t a deal — it’s just words he feels free to ignore a few days later.

The story so far: Two weeks ago, Trump declared that if Congress came up with a plan to protect Dreamers — undocumented immigrants brought here as children — while enhancing border security, he would sign it. Two days later, a bipartisan group of senators brought him a plan doing just that — and he rejected it, complaining about immigrants from “shithole countries.”

On Friday, Chuck Schumer, the Senate Democratic leader, seemed to have at least a short-term agreement with Trump, only to see it pulled back a few hours later. Working with Trump is “like negotiating with Jell-O,” Schumer fumed.

Finally, on Monday Democrats agreed to a three-week extension of funding in return for a promise from Mitch McConnell, the Senate majority leader, of a vote on immigration legislation (we’ve heard nothing from Trump). If this vote doesn’t happen, we’ll be back to square one Feb. 8. Anyone want to take bets?

Basically, then, the government of the world’s greatest nation is lurching from crisis to crisis because its leader can’t be trusted to honor a deal. But what did you expect? Trump’s whole business career has been a series of betrayals — failed business ventures from which he personally profited while others, whether they were Trump University students, vendors or creditors, ended up holding the bag. And he hasn’t grown a bit in office, unless you count that mysterious extra inch.

There are two things you need to realize about Trump’s utter unreliability. First, it has ramifications that go far beyond the recent shutdown. Second, it’s made possible, or at least much worse, by his enablers in Congress.

Think, for example, about the international consequences of a U.S. president whose word can’t be trusted. Who can we count on to be a reliable ally, when no country knows whether America will stand by it if it needs help?

So far, at least, financial markets continue to regard the U.S. government as trustworthy, even though during the 2016 campaign Trump openly talked about forcing the nation’s creditors — like creditors of some of his businesses — to accept less than they were owed. But does this government have any reserve of financial credibility if something should go wrong? Probably not.

In other words, Trump’s unreliability is a big problem, over and above the substance of his policies. But here’s the funny thing: While his instincts are clearly autocratic, the Constitution doesn’t set him above the law. Congress has the power to constrain his actions, to force him to honor promises. His ability to keep betraying those who trust him depends entirely on the willingness of Republicans in Congress to go along.

For example, any two of the Republican senators currently wringing their hands over the betrayal of the Dreamers could have forced action by withholding their votes on the Trump tax cut. They didn’t. Similar inaction explains why Trump has been able to violate all previous norms against exploiting his office for personal gain, and much more.

The result is that promises from the U.S. government are now as worthless as those from a tinpot dictator. We don’t yet know how high a price we’ll pay for that loss of credibility, but it probably won’t be small.